1. Technical Field of the Invention
The present invention relates generally to test and measurement systems, and in particular, to network analyzers for measuring the antenna return loss of cellular base station antennas.
2. Description of Related Art
Network analyzers measure the antenna return loss of a cellular base station antenna by injecting a swept signal covering the antenna transmit and/or receive frequencies into the device under test (DUT), i.e., antenna, and measuring the magnitude and phase of the signal that is reflected back. For example, typically, a technician connects the network analyzer to the feeder cable extending between the antenna and the base station, and injects a signal into the feeder cable. If there are any discontinuities in the feeder cable or antenna, part of the signal may be reflected back down the feeder cable to the network analyzer.
Network analyzers are primarily utilized when the antenna being tested is not currently in use. However, if a “live” (i.e., currently in-use) test is required, the injected signal has the potential to disrupt the existing radio links between the base station and customers' mobile phones. For example, when testing a receive antenna (i.e., an antenna operating at the base station receive frequencies), as the network analyzer's source sweeps through the channel that the mobile phone's transmitter occupies (i.e., up-link channel from the mobile phone to the base station), a high level of interference is experienced at the input to the base station receiver. The interference could result in a reduction of the call quality, and possibly cause the call to drop.
One known way to avoid the interference that results from introducing a swept signal onto a “live” antenna is to use the base station transmitter as the source stimulus. Instead of injecting a swept signal into the DUT, a device, such as a Voltage Standing Wave Ratio (VSWR) monitor, can be connected to the feeder cable extending from the base station transmitter to the antenna to measure the actual transmitted signal (i.e., the signals that are transmitted on the down-link frequencies from the base station transmitter to the customer's mobile phones), and the amount of the transmitted signal that is reflected back from the antenna. Since there are no extraneous signals injected onto the antenna, the potential for interference is eliminated.
However, most base station receivers are connected to different physical antennas than base station transmitters. Therefore, by using the base station transmitter as the source, in many cases, only the transmit antennas can be tested. The receive antennas would have to be tested using a swept signal, which produces interference, as discussed above.
Additionally, most antenna customers want to know the antenna return loss over the entire transmit frequency band to make an informed decision about the status of the antenna (e.g., return loss degradations at only some of the frequencies may indicate a slowly degrading antenna that is destined to fail and should be replaced). However, by using the base station transmitter as the source, transmitted and reflected signal measurements can only be made on the frequencies at which the base station is actually transmitting. Furthermore, without a broadband return loss measurement, the time-domain impulse response of the transmit antenna cannot be accurately calculated. The time-domain impulse response is used by time-domain reflectometry (TDR) to locate the physical position of breaks in the antenna cable. To be effective, TDR requires a broad frequency sweep.
Another known technique for avoiding interference that results from introducing a signal onto a “live” antenna is to modulate the network analyzer's source to generate a spread-spectrum signal. For example, both frequency chirps and direct-sequence spread spectrum modulation methods have been used. In order to guarantee that the power spectral density of the source falls below that of the received mobile signal, wide spreading of the source is required. As a result of the source frequencies being spread, the frequency resolution of the resulting measurement is degraded. In addition, direct sequence spread spectrum modulation has the additional disadvantage of being relatively expensive to implement. What is needed is a technique for measuring antenna return loss of cellular base station antennas without disrupting the existing traffic on the network.